Methacrylic resins are transparent and have characteristics that they have higher light transmittance, weather resistance and rigidity than other transparent plastic resins, and are used in a wide range of applications including automotive parts, lighting apparatus, building materials, signboards, nameplates, paintings and windows for display devices. Recently, more and more applications require complicated molding. For example, in injection molding of large and thin molded articles, poor flowability of resin leads to an insufficient injection pressure, resulting in failure to mold or causing significant deformation of molded articles. For this reason, high flowability resins that can be molded even at low injection pressure are desired. At the same time, the appearance, mechanical strength such as solvent resistance and heat resistance are required not to be deteriorated.
So far, as a known method for improving the mechanical strength and moldability of methacrylic resins, a method is generally known in which flowability is provided using a methacrylic resin having a low molecular weight and mechanical strength is provided using a methacrylic resin having a high molecular weight or a slightly crosslinked structure. In this connection, techniques of melt-mixing a high molecular weight or low molecular weight methacrylic resin or widening molecular weight distribution using a branched structure have been reported (see, for example, Patent Documents 1, 2 and 3).
However, in the methacrylic resin described in Patent Document 1, two methacrylic resins having a different molecular weight are only mixed, and high flowability and mechanical strength are not simultaneously satisfied. Further, Patent Document 2 discloses a technique of copolymerization of a large amount of another vinyl monomer copolymerizable with methyl methacrylate and a methacrylic resin of a low molecular weight. However, the resulting methacrylic resin has insufficient flowability.
The process for producing a slightly crosslinked methacrylic resin using a multifunctional monomer described in Patent Document 3 has a problem that control of the multifunctional monomer is extremely difficult. When the amount of the multifunctional monomer is too large, homogeneity in mixing is reduced and the appearance of molded articles is deteriorated. On the other hand, when the amount of the multifunctional monomer is too small, the advantage of improvement of flowability and maintaining mechanical strength is not found.
Patent Document 1: JP-B-1-22865
Patent Document 2: JP-A-4-277545
Patent Document 3: JP-A-9-207196